Unitarian Universalism emerged from two separate denominations: Unitarianism and Universalism.
Originally, all Unitarians were Christians who didn't believe in the Holy Trinity of God (Father, Son, and Holy Ghost). Instead, they believed in the unity, or single aspect, of God. Unitarianism eventually began to stress the importance of rational thinking, each person's direct relationship with God, and the humanity of Jesus.
Unitarians have been very influential throughout American history, especially in politics and literature. Some famous Unitarians include Presidents John Adams and John Quincy Adams, Louisa May Alcott, Ralph Waldo Emerson, Paul Revere, President William Howard Taft, and Frank Lloyd Wright.
While Unitarian beliefs have been around since soon after Jesus died, people didn't form religious groups based on the ideas until the middle of the fifteen hundreds in Transylvania and the middle of the sixteen hundreds in England. The religious authorities of the times saw these early Unitarians as heretics and often persecuted them. Important figures from this period in Unitarian history include John Biddle, Francis David, Michael Servetus, King John Sigismund and Faustus Socinus.
Unitarianism flourished in the religious freedom of early America. By 1825 Unitarian ministers had formed a Unitarian denomination called the American Unitarian Association.
Speaking out on issues such as peace, education reform, prison reform, orphanages, capital punishment, moderation in temperance, ministry to the poor, and the abolition of slavery, the AUA's liberal voice was soon heard throughout the country. The influential Unitarians from this era included William Ellery Channing, Theodore Parker, Joseph Priestly, and Thomas Starr King, who was also a Universalist.
American Unitarianism went through many changes over the next 150 years, from the introduction of Transcendentalist thought in the middle of the eighteen hundreds through debates about war and pacifism in the Civil War and the two World Wars to the influx of Humanism in the early 1930s. These changes slowly made Unitarianism a more broad and flexible faith.
After growing increasingly theologically and ethically close, the Unitarian and Universalist denominations consolidated in 1961 to form the new religion of Unitarian Universalism.
Unitarian Universalism no longer solely holds traditional Unitarian or Universalist beliefs, but does draw directly on its heritage for much of its inspiration and grounding.
There are many Unitarian congregations today outside the United States that are part of the Unitarian Universalist community. The largest concentrations of Unitarians outside the United States are in Transylvania (now part of Romania and Hungary) and India.
There are also Unitarian organizations that are not affiliated with Unitarian Universalism, most of which call themselves Biblical or Christian Unitarians. You can search the internet to find more information on these groups.